[44] With respect to the "relevance" criterion, Mohanstates that the judge must conduct a cost-benefit analysis to determine "whether its value is worth what it costs" (p. 21, quoting McCormick on Evidence (3rd ed. 1984), at p. 544). The cost-benefit analysis requires the judge to balance the probative value of the evidence against its prejudicial effect (Mohan, at pp. 23-24).

[45] As for the "necessity" criterion, Mohan holds that "[i]f on the proven facts a judge or jury can form their own conclusions without help, then the opinion of [an] expert is unnecessary" (p. 23, quoting Lawton L.J. in R v. Turner, [1975] 1 Q.B. 834, at p. 841). The Court went on to note that the concern "inherent in the application of this criterion [is] that experts not be permitted to usurp the functions of the trier of fact" (p. 24).

[46] Given the concerns about the impact expert evidence can have on a trial — including the possibility that experts may usurp the role of the trier of fact — trial judges must be vigilant in monitoring and enforcing the proper scope of expert evidence. While these concerns are perhaps more pronounced in jury trials, all trial judges — including those in judge-alone trials — have an ongoing duty to ensure that expert evidence remains within its proper scope. It is not enough to simply consider theMohan criteria at the outset of the expert's testimony and make an initial ruling as to the admissibility of the evidence. The trial judge must do his or her best to ensure that, throughout the expert's testimony, the testimony remains within the proper boundaries of expert evidence. As noted by Doherty J.A. in R. v. Abbey, 2009 ONCA 624, 97 O.R. (3d) 330, at para. 62:

The admissibility inquiry is not conducted in a vacuum. Before deciding admissibility, a trial judge must determine the nature and scope of the proposed expert evidence. In doing so, the trial judge sets not only the boundaries of the proposed expert evidence but also, if necessary, the language in which the expert's opinion may be proffered so as to minimize any potential harm to the trial process. A cautious delineation of the scope of the proposed expert evidence and strict adherence to those boundaries, if the evidence is admitted, are essential. The case law demonstrates that overreaching by expert witnesses is probably the most common fault leading to reversals on appeal. [Emphasis added; citations omitted.]

[47] The trial judge must both ensure that an expert stays within the proper bounds of his or her expertise and that the content of the evidence itself is properly the subject of expert evidence.